Image orthicon with a narrow range of electron energy in the scanning beam



Sept. 4, 1962 R. K. H. GEBEL 3,052,807

IMAGE ORTHICON WITH A NARROW RANGE OF ELECTRON ENERGY IN THE SCANNING BEAM Filed Jan. 29, 1960 2 Sheets-Sheet l T T a.

INVENTOR. R. K. H. GEBE BY ab...

ATTORNEY AGENT R. K. H. GEBEL 3,052,807 IMAGE ORTHICON WITH A NARROW RANGE OF ELECTRON Sept. 4, 1962 ENERGY IN THE SCANNING BEAM 2 Sheets-Sheet 2 Filed Jan. 29, 1960 ZOJ I BY Wm AGENT INVENTOR. R. K.H. G EL ATTORNEY United rates Patent 3,052,807 IMAGE ORTI-HCON WITH A NARROW RANGE OF ELECTRON ENERGY IN THE SCANNING BEAM Radames K. H. Gebel, Dayton, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Filed Jan. 29, 1960, Ser. No. 5,570 3 Claims. (Cl. 313-82) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

A low beam modulation factor impairs the performance of image orthicon tubes at low light levels. The wide energy distribution of the electrons in the scanning beam of a conventional tube is one of the reasons for low beam modulation since, at low light levels, the information on the target plate consists of such a small charge that only the very fastest electrons in the beam reach the target plate and are effective as charge neutralizers. Since these electrons produce the beam modulation, the percentage modulation is low.

' In accordance with the invention, the beam modulation percentage is increased by limiting the energy of the ,bearns electrons to a very narrow range. Briefly, this is accomplished by restricting the electrons emitted from the cathode to a very narrow beam at an angle to the longitudinal axis of the tube. The transverse components of the electrons velocities together with the axial magnetic field of the tube cause the electrons to travel along helical paths having different diameters depending upon the initial energies of the electrons. A barrier having a small aperture is then placed to intercept all electrons except those traveling along the helix having the diameter estab- 'l-ished by the position of the aperture, or helices having diameters very close to this value. The electrons passing through the aperture will therefore have substantially the 'same energies.

A more detailed description of the invention will be given with reference to the accompanying drawings in which FIG. 1 is a fractional view showing the essential features of the invention,

FIGS. 2, 2a, 3 and 3a are projections of the helical electron path of FIG. 1,

FIG. 4 is -a diagram illustrating certain geometric relationships in FIG. 1, and

FIG. 5 is a partial view of an image orthicon incorporating the invention.

Referring to FIG. 1, 1 is a cathode cylinder having a heating element 2 and, on its end, a small pellet 3 of an etficient thermionic emitter such as barium oxide. Two closely spaced metallic plates 4 and 5 are positioned normal to the axis z, which may be the longitudinal axis of an image orthicon tube, with plate 4 adjacent to emitter 3. Plates 4 and 5 have small apertures 6 and '7 of, for example, 0.1 mm. diameter in alignment with the axis 8 of the cathode. There is also provided a third metallic plate 9 parallel to plate 5 and at a distance /2 therefrom. Plate 9 has a small aperture 10, which may be of the same size as apertures 6 and 7, having its center in the xz plane at a distance d from the z axis. Plate 4 has an adjustable negative potential relative to the cathode,

3,52,87 Patented Sept. 4, 1962 provided by potentiometer 111, and serves as a means for controlling the beam intensity. Plates 5 and 9 are at the same positive potential relative to the cathode, which potential is'supplied by potentiometer 12. The vector 13 represents the axial magnetic field normally provided for in image orthicons.

Electrons emitted from the emitter 3 of the cathode pass through aperture 6 and are accelerated toward aperture 7 by the positive field produced by positive plate 5. The electrons pass through aperture 7 and into the space between plates 5 and 9 with a velocity determined by the potential of plate 5 relative to the cathode. As a result of the tilt of the cathode the velocity vector of the electrons passing through aperture 7 makes an angle on with respect to the z axis. Consequently the velocity vector of the electrons has a component parallel to the tube axis 2, and magnetic field vector B, and a component transverse to the tube axis and the magnetic field vector B. These two components result in a helical movement of the electron through the space between plates 5 and 9 as represented by the path 13. The yz, x-z and y-x projections of path 13 are shown in FIGS. 2, 3, 2a and 3a. The diameter d of the helix 13 for each electron, as will be seen later, is determined by the kinetic energy of the electron when it emerges from aperture 7, and the angle a.

With proper selection of the distance /2 all electrons within a very narrow predetermined range of kinetic energies will have helical paths of substantially the same diameter d and will pass through aperture 10. Electrons having kinetic energies above or below this range will have helical paths of greater or lesser diameter and be intercepted by plate 9. Consequently, the stream of electrons passing through aperture 10 and continuing in the axial direction have a very narrow range of kinetic energies as desired for the scanning beam of an image orthicon operating at low light levels.

The values of d, the diameter of helix 13, and l, the lead of the helix are determined as follows:

(1) d=-= 7 n a Sin a 1.759Xl0 B sin a where =electron change to mass ra.tio=l.759 10 emu.

m (approx.) 7

(2) v= /2%10m=59.36 10 W cmJsec.

where u=energy of electrons in electron-volts emerging from aperture 7 (equals initial energy of electrons plus potential at aperture 7).

substituting Equation 2 in Equation 1 surface, as shown in FIG. 4, the helix becomes a straight line, forming the hypotenuse of a right triangle. The angle a between B and the electron velocity vector in FIG. 1 is identical to the angle a in FIG. 4; therefore, the path length L traveled by the electrons in one turn of the helix is given by 1rd (4) sin a From FIG. 4 it is also seen that (5) l=L cos a substituting the value of L from Equation 4 in Equation 5 gives (6) l=1rd cos a considering a specific example, assume B=13.4 Gauss w=3 volts then, from Equation 3,

d (am/s dm,,=d+ 1.005 cm.

dmi =d .995 cm.

and

mitt 69 6 Then, from the rearrangement of Equation 3 The energy module u min d sin 01 a max di sin C(mnx mod d i S min where ines min Ad Sin (a +Aa) Os Aa+ sin 2A0l sln 04min tan (1 sin Aa sin 2A0 21 f A tan Otmiu 1781 cu (1 (2) Hence Ad 2 sin 2Aa min) (1+ta4n min or, for the specific case,

Or, in other words, the energy distribution of the electrons leaving aperture 10 has an absolute range of 5%, or differs il /2% from the average value.

FIG. 5 illustrates a possible arrangement of an image orthicon tube incorporating the invention. The conventional image ort-hicon is well known in the art and described in the literature; for example, in an article entitled The Image Orthicon by Rose, Weimer and Law, appearing in the July 1946 issue of the Proceedings of the Institute of Radio Engineers. Only the cathode end of the tube is shown in FIG. 5. Elements are numbered to correspond to FIG. 1. The plate 9', which is the first dynode of the tubes electron multiplier, may also serve the function of plate 9 in FIG. 1. The beam 13 is caused to scan the target electrode by sweep yoke 20 in conventional manner. The electrons in excess of those required to neutralize the charge on the target return along path 21 to first dynode 9' of the electron multiplier, the remaining dynodes and output electrode of which are indicated at 22, 23, 24, 25 and 26, respectively. Coil 27 provides the axial magnetic flux B.

I claim:

1. Apparatus for use in a cathode ray tube for producing an electron beam having a very narrow range of electron energies, said apparatus comprising: a pair of parallel spaced metallic equipotential plates; means for producing a constant magnetic field normal to said plates in the space between said plates; means for injecting a narrow beam of electrons into said space through a small aperture in one of said plates, said beam making an acute angle with said magnetic field whereby each injected electron travels through said space along a helical path having a diameter and lead determined by the electron velocity; and a small circular aperture in the other of said plates so positioned as to pass only those electrons whose paths substantially coincide with a single helical path, whereby the electrons passing through the aperture have substantially the same velocities.

2. Apparatus for use in a cathode ray tube for producing an electron beam having a very narrow range of electron energies, said apparatus comprising: an electron gun structure designed to produce a narrow beam of electrons at an acute angle to a predetermined reference axis, said gun structure comprising an electron emitting cathode, a beam intensity control electrode in the form of a metallic plate positioned close to said cathode and having a small aperture through which a portion of the electrons emitted by said cathode pass, means for maintaining said beam intensity control electrode at an adjustable negative potential relative to the cathode, an accelerating electrode in the form of a metallic plate parallel to and close to said beam intensity control electrode and having an aperture of substantially the same size as the aperture in the beam intensity control electrode, means for maintaining said accelerating electrode at a postiive potential relative to said cathode, said plate being normal to said reference axis, and said cathode and said apertures being in alignment, with the axis of alignment intersecting said reference axis at the center of the aperture in said accelerating electrode and making an acute angle with said reference axis; a third metallic plate normal to said reference axis and spaced a predetermined distance from said accelerating electrode; means for maintaining said third plate at the same potential as said accelerating electrode; means for establishing a magnetic field parallel to said reference axis in the space between said accelerating electrode and said third plate, whereby each electron entering this space through the aperture in said accelerating electrode travels a helical path the diameter and lead of which depends upon the electron velocity; and a small aperture in said third plate so positioned as to pass only those electrons Whose paths substantially coincide with a single helical path whereby the electrons leaving said aperture have substantially the same velocities.

3. Apparatus as claimed in claim 2 in which the center 6 of the aperture in said third plate lies at a distance d from velocity vector of the electrons entering the space between said reference axis in a plane containing said reference axis said accelerating electrode and said third plate through the and normal to the plane defined by said reference axis aperture in said accelerating electrode. and said axis of alignment, and in which the spacing between said accelerating electrode and said third plate 5 References Cited In the file of this Patent equals d UNITED STATES PATENTS 2,429,55s Marton Oct. 21, 1947 2 2,500,455 Fisher Mar. 14, 1950 2,638,561 Sziklai May 12, 1953 where a is the angle between the magnetic field and the 10 2,310,091 Harsh Oct 15, 5 

